Composite coaxial coupling device and coaxial window



June 3, 1969 E. J. COOK 3,448,331

COMPOSITE COAXIAL COUPLING DEVICE AND COAXIAL WINDOW Filed July 19, 1966Sheet of :5

FIG. 2 W4- I NVENTOR.

I2 EDWARD J. 000K- ATTORNEY June 3, 1969 E. 'J. COOK 3,448,331

COMPOSITE COAXIAL COUPLING DEVICE AND COAXIAL WINDOW Filed July 19, 1966Sheet of s n, /2 A I FA w T v 4 l 7 L 7:100 pC J BEAM i/ 3 C E ,7 E I L1 B 7 G 4 FIG. 5

FIG. 7

/ J\ V I// I NVENTOR.

EDWARD J. 000K BY a ATTORNEY June 3; 1969 E. J. COOK 3,448,331

COMPOSITE COAXIAL COUPLING DEVICE AND COAXIAL WINDOW Filed July 19, 1966Sheet of 3 FIG. 8

FIG. 9

FIG.|I

INVENTOR.

ATTORNEY United States Patent 3,448,331 COMPOSITE COAXIAL COUPLINGDEVICE AND COAXIAL WINDOW Edward J. Cook, South Hamilton, Mass.,assignors to Varian Associates, Palo Alto, Calif., a corporation ofCalifornia Continuation-impart of application Ser. No. 525,455, Feb. 7,1966. This application July 19, 1966, Ser. No. 574,271

Int. Cl. H01j /50; H0311 7/38; H01p 1/00 U.S. Cl. 315-3953 8 ClaimsABSTRACT OF THE DISCLOSURE A composite coaxial coupling device andcoaxial wave permeable Window for microwave structures includingmicrowave tubes is disclosed. The coaxial coupling device and windowincludes an inner conductor portion which projects into a fieldcontaining a region of space defined between a pair of conductors. Adielectric wave permeable window member coaxially surrounds the innerconductor and projects into the field containing region of space. In oneembodiment, the field containing region of space is defined by a wavesupporting structure having a radial line portion and a coaxial portionwith the outer wall of the coaxial portion apertured for passage of thecenter conductor of the coupler and wherein a portion of one wall of theradial portion of the line forms an overlying outer conductor extensionportion for a coaxial coupling device to load the coaxial coupler tomaintain the characteristic impedance to obtain a broadband match. Inanother embodiment of the present invention, a coupling capacitorstructure is disposed at the inner end of the inner conductor of thecoaxial line, whereby the capacitor structure balances out a smallseries inductive reactance of the inwardly projecting portion of thecoaxial line to provide a refiectionless window.

The present invention is a continuation-in-part of parent applicationSer. No. 525,455, filed Feb. 7, 1966 now abandoned and assigned to thesame assignee as the present invention.

Heretofore, composite coaxial line coupling devices and RF. coaxial linewindows have been used. However, in these prior devices, the windowtypically took the form of a ceramic or glass disk or bead disposedacross the coaxial line at its entrance into the inductor of thecircuit, usually a cavity resonator, to which the center conductorportion was connected or coupled. In such a device, the windowintroduces an impedance mismatch in the coaxial line which makes itunsuited for broadband coupling applications where wave reflections fromthe coupling device are to be avoided over a very broad frequenceyrange, for example, of 4 to 14 gc.

In the present invention, a composite coaxial coupling device andcoaxial window is provided which is essentially refiectionless over theaforementioned frequency range. The broadband characteristic isattributable to utilizing a frame member and a portion of the window asintegral portions of the coaxial line coupling device, whereby thedistance betwen the anode circuit and the window are reduced to aminimum to obtain broadband operation.

The principal object of the present invention is to provide an improvedcomposite coaxial coupling device and window for a coaxial line andtubes using same, whereby improved broadband performance characteristicsare obtained.

One feature of the present invention is the provision of a compositecoaxial line coupling means and cylindrical dielectric window whereinthe window surrounds the cen- 3,448,331 Patented June 3, 1969 terconductor of a coaxial line and projects with the center conductor intothe region of the circuit to be coupled to the line with an extension ofthe outer conductor of the coaxial line disposed adjacent the inwardlyprojecting cylindrical window member to maintain its characteristicimpedance whereby reflections are avoided over a broadband.

Another feature of the present invention is the same as the precedingfeature including the provision of a disk shaped frame member portionsealing the center conductor of the coaxial line to the inwardlyprojecting cylindrical window at its inner end, and wherein the framemember is closely spaced and coupled to a conductor portion disposedOpposite to the conductor portion through which the coaxial lineprojects, whereby the impedance match is enhanced.

Another feature of the present invention is the same as one or more ofthe preceding features wherein the outer end of the cylindrical windowis sealed over an axially coextensive region to a coaxially disposedcylindrical frame member which surrounds the cylindrical window, andwhich cylindrical frame forms a substantial length of the outerconductor of the coaxial line.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the composite window and coaxialcoupling means is used for coupling wave energy from a voltage tunablemagnetron to a load.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the center conductor of the coaxialline, which projects into the region of the circuit to be coupled, iscapacitively coupled to the circuit at its inner end portion; wherebythe capacitance serves to resonate a series inductance of the protrudingcenter conductor of the coaxial line to better the impedance matchbetween the coaxial line and the circuit being coupled.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic transverse sectional view of a magnetronoscillator coupled to a load,

FIG. 2 is an w-fl diagram showing the disperson characteristics for avoltage tunable magnetron,

FIG. 3 is a longitudinal sectional view of a magnetron incorporatingfeatures of the present invention. The view being similar to that astaken along line 3-3 in the direction of the arrows of FIG. 1.

FIG. 4 is an enlarged front view of an interdigital line magnetroninteraction circuit similar to that as seen by a view taken along line44 of FIG. 3 in the direction of the arrows,

FIG. 5 is a schematic equivalent circuit diagram for the interdigitalmagnetron interaction circuit of FIG. 3,

FIG. 6 is a transverse schematic circuit diagram for the tube of FIG. 3,

FIG. 7 is an enlarged sectional detailed view of an alternativeembodiment of the present invention as delineated by lines 7-7 of FIG.3,

FIG. 8 is an alternative embodiment of a portion of the structure ofFIG. 3 delineated by line 8-8,

. FIG. 9 is a transverse sectional view of a portion of the structure ofFIG. 8 taken along line 9-9 in the direction of the arrows,

FIG. 10 is a schematic equivalent circuit diagram for the outputcoupling circuit of FIGS. 8 and 9, and

FIG. 11 is an alternative embodiment of the structure of FIG. 7.

Referring now to FIG. 1 there is shown in schematic form a voltagetunable magnetron. More specifically, the magnetron includes an anodestructure 1 coaxially surrounding a cathode electrode 2 and defining, inan annular region therebetween, a magnetron interaction region 3. Theanode 1 is provided with a suitable fundamental backward wave periodicwave circuit 4 formed therein facing the cathode 2 for interaction withthe electrons of an electron stream in the magnetron interaction region3. In operation suitable operating potentials are applied betweencathode 2 and anode 1 in the presence of an axially directed magneticfield B in the magnetron interaction region 3 for cumulative interactionwith the electron stream to produce an output signal. The output signalis extracted from the circuit 4 via suitable coupling means 5 and fed toa load 6. The voltage tunable magnetron will oscillate at a frequencydetermined by the circuit 4 and by the voltage between the cathode 2 andthe anode 1. The frequency range over which the tube may be tuneddepends upon the loaded Q of the periodic circuit 4. Circuit 4 may beloaded by heavily coupling the circuit 4 to the load 6 or the circuitmay be provided with internal resistance to lower its load Q.

Referring now to FIG. 2 there is shown a dispersion characteristic for atypical fundamental backward wave circuit. When the circuit 4 isre-entrant, such that wave energy can travel in a continuous path aroundthe circuit, the dispersion characteristic becomes discontinuous in thesense that a number of resonant modes of oscillation are producedcorresponding to an integral number of full wavelengths of wave energytravel around the circuit 4. Typically, there will be N/Znumber ofresonant modes established where N is the number of periodic elementstaken around the circuit 4. Thus, if a circuit has 8 slot resonators asdepicted in FIG. 1, there will be N/2 or 4 resonant modes of oscillationestablished at equal intervals of B or phase shift per period of thecircuit. Heretofore, it has been the practice to operate on the 1r modedesignated N/Z on the diagram. However, in the tube incorporating theoutput coupling feature of the present invention, the (N/2-1) mode isused, which, for a given operating frequency band, permits the anodecircuit 4 to have larger dimensions and thus greater power handlingcapability than would be attainable if the tube was designed foroperation on the 1r mode. Thus, the tube is operable over an electronicbandwidth from w to ta corresponding to synchronous beam voltages of Vto V as shown in the diagram of FIG. 2. However, in order to obtain sucha wide tuning range, the circuit 4 must be heavily loaded such that theloaded Q of the anode circuit is on the order of to 20. If the couplingto the load does not provide a good match between the load and the tube,the tuning range of the tube will be drastically affected. Anyreflection introduced in the coupling between the load and the tubediminishes the coupling thereby raising the Q of the anode circuit 4 andreducing its tunable bandwidth. Thus, a reflectionless match between theanode circuit 4 and the load is desired for maximum tunable bandwidth.

Referring now to FIG. 3, there is shown in longitudinal section avoltage tunable magnetron incorporating the output coupling circuit ofthe present invention. More specifically, the tube includes an anode 1having a crown supported interdigital type anode circuit 4 essentiallyas shown in FIG. 4. The interdigital anode circuit 4 is crown supportedby a pair of annular plates 11 which include radial portions followed byaxial directed portions and which are closed at their outer ends by endwall 12. The anode circuit coaxially surrounds a cold cathode soleelectrode 2 operated at cathode potential. An annular magnetroninteraction region 3 is defined between the interdigital circuit 4 andthe axially coextensive portion of the cathode electrode 2. An electrongun 13 is disposed at the opposite end of the tube from the cold cathodeelectrode 2 and includes a helical filamentary emitter 14 surrounded byan injector electrode 15 operating at a substantially lower potentialthan the anode circuit 4 whereby the electrons are axially injected intothe magnetron interaction region 3 under the confining effect of theaxial magnetic field B.

The vacuum envelope for the tube is defined by several annular memberssealed together in a vacuum tight manner at their abutting surfaces. Thecathode sole 2 is sealed to an annular ceramic insulator 16 which inturn is sealed to the lower conductor plate 11 of the anode. Lower plate11 is also sealed in a vacuum tight manner to the other plate 11 via theintermediary of the coaxial axially extending wall portions and the endclosing wall 12. The upper side of the upper annular anode plate 11 issealed to another insulating ring 17 as of alumina which is in turnsealed to end enclosing plate member 18 as of ceramic.

The equivalent circuit for the anode circuit 4 is shown in FIG. 5. Thecircuit is essentially a two wire transmission line with series loadingin the two conductors A and B formed by the quarter wave slot resonatorsformed between adjacent fingers in each of the lines. This can bereadily seen by reference to FIG. 4 wherein one of the plates 11 isdesignated as A and the opposed plate 11 is designated as B. The uppercutoff frequency of the circuit is formed when the slot resonatorsformed in the interdigitated conductors A and B become resonant. Thiscorresponds to a length 1 between successive interaction regions of ahalf wavelength and a finger length of nearly a half wavelength. On thew-fi diagram this upper cutoff frequency is :1 and for, the tube of FIG.3 this frequency is selected to be approximately 70 gc. corresponding toa finger length of approximately 0.084".

The low frequency cutoff of this circuit 40;; corresponds to a resonanceof the shunt elements L and C formed by the folded radial quarter wavecavity having a length designated 1 in FIG. 3 and capacitively loaded bythe capacitance C between adjacent fingers of the interdigital line. Inthe tube of FIG. 3 I is selected such that the resonant frequency of the11' mode corresponding to (0 is approximately 4 gc. With this lowfrequency cutoff, substantial mode separation is obtained between the(N/Z-l) operating mode at 856 to 9.6 gc. and

the adjacent 1r mode. The electron stream in the magnetron interactionregion 3 successively cumulatively interacts with the electric field Ebetween adjacent fingers which corresponds to the capacitive voltagesdeveloped across capacitors C in FIG. 5.

At the operating (N/Z-l) mode the shunt cavity resonator portion of thecircuit formed by the shorted folded section of radial transmissionline, defined by annular radial and axial plates 11 as closed by endwall 12, operates in the TM mode. This mode pattern is shown in FIG. 6.

The output signal is coupled out of the cavity via coaxial couplingmeans 21 which couples across a portion of the shunt inductor L Morespecifically, a coaxial line 22 is coupled to a load, not shown, and inthe embodiments of FIGS. 3 and 7 has its center conductor 23conductively connected to the inner conductor plate 11 of the crownsupported interdigital line by passing through an aperture 24 in theouter wall 11. A cylindrical dielectric window member as of aluminaceramic 25 coaxially surrounds the conductor 23 and projects inwardly ofthe vacuum envelope into the region of space 26 defined between the twoconductor portions 11. A disk shaped metallic frame member 27 as ofcopper plated molybdenum is brazed to the end of the inner end of thecylindrical window member 25 in a vacuum tight manner. The frame member27 is centrally apertured and is brazed to a molybdenum plug 28 which inturn is brazed to conductor portion B of the cavity. Plug 28 iscentrally bored and tapped to receive the center conductor 23 threadedtherein. The outer end of the cylindrical window 25 is brazed to acoaxially surrounding sleeve 29 as of nickel over an annular axiallycoextensive brazed region 31 to form a vacuum tight seal therebetween.Sleeve 29 '5 forms the outer conductor of the coaxial line segment,which includes the window member 25 and the sleeve 29 and is in turnbrazed in a vacuum tight manner at its inner end to the apertured wall24 of conductor 11.

The cylindrical window member 25 is closely spaced to the radialconductor portion 11 such as to maintain its characteristic S052impedance for that portion of its length which projects into the cavity26. This closely spaced outer conductor region is designated as 32 andassures a reflectionless coupling to the fields of the cavity bymaintaining the characteristic impedance of the line 23 into the pointof connection between plug 28 and conductor 11. An externally threadedminiature coaxial connector adapter 34 is soldered over the outer end ofthe sleeve 29 to facilitate connection of the coaxial line 23 to astandard 509 miniature coaxial line.

By designing the window assembly 21 such that it presents substantiallythe same characteristic impedance, i.e. 509 as the miniature coaxialline into the point where it is connected to the inner conductor 11there is assured a refiectionless connection between the circuit 4 andthe load resistance of the load 6, not shown in FIG. 3, whereby thecircuit is heavily loaded by the load and its Q reduced to approximately-20 for broadband operation.

Referring now to FIG. 7 there is shown an alternative embodiment of thepresent invention wherein the closely spaced conductor region 32 hasbeen replaced by a continuation formed by an inward extension of thesleeve 29 designated at 35. Thus, as in the embodiment of FIG. 3, thewindow segment maintains the characteristic 509 impedance into the pointof connection of the inner conductor 23 to the axially directed portionof plate 11 via the intermediary of frame member 27.

Referring now to FIGS. 8-11 there is shown an alternative embodiment ofthe present invention. It has been found that in the output couplingarrangements of FIGS. 3 and 7, that the center conductor segment of thecoaxial line 21, which projects into the circuit being coupled to, hasassociated therewith a small amount of series inductance Ls, see FIG.10. This inductance Ls presents a small reactance tothe slow wavecircuit 4 which tends to shift the operating frequency of the tube andto slightly decouple the load from the tube 1.

This small series industance Ls is balanced out in the embodiments ofFIGS. 811 by capacitively coupling the inner end of the center conductor23 to conductor B of the slow wave circuit 4. The series capacitance ofthe coupling capacitor Cs is series resonated at the frequency of thetube with the series inductance Ls. Due to the heavy coupling of thecircuit 4 to the 50S load the Q of the series resonant coaxial couplingcircuit, as coupled to the slow wave circuit 4, is so low that the passband of the series resonant coupling circuit is much wider than theelectrically tunable band of the tube on the operating (N/2-1) mode.

The apparatus of FIGS. 8-1l is essentially identical to the apparatus ofFIGS. 3 and 7 except for the provision of the coupling capacitor Csprovided at the inner end of the center conductor 23. More particularly,in the structure of FIGS. 8 and 9 the plug 28 is terminated flush with acircumferentially enlarged window frame member 27. A conductive tab 41as of copper is brazed to conductor B such as to provide a portionfacing the frame member 27 which is substantially of the same dimensionsas the window frame member 27. The frame member 27 is then slightlyspaced from the tab 41 to form the series coupling capacitor structureCs.

In a typical example of the capacitive coupling structure the tab 41 ismade to have a circumferential extent of about 0.250", a thickness of0.030", and a height h of about 0.100. The spacing s between the opposedfaces of the tab 41 and the end of the frame 27 is dimensioned forseries resonance at the center of the pass band of the tube 1 andtypically falls within the range of 0.015" to 0.030".

Referring now to FIG. 11 this structure is essentially identical to thestructure of FIG. 7 except that the window frame member 27 is terminatedshort of the opposed conductor B by a slight spacing s to form theseries coupling capacitor Cs. The spacing s typically falls within therange of 0.015" to 0.030".

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A composite coaxial line coupling device and gas tight wave permeablewindow assembly including, an evacuated wave supporting region of spacedefined be tween a pair of conductive portions defining a section ofwave supporting structure having a radial line portion and a coaxialaxially directed portion and containing fields to be coupled to acoaxial line, means forming a coaxial line having inner and outerconductor portions for coupling to the fields of said field containingregion of space and having a certain characteristic impedance, the outerwall of said coaxial portion of said section of wave supportingstructure being apertured for passage of said inner conductor of saidcoaxial line therethrough for coupling to the other conductor portion ofsaid wave supporting structure, means forming a wave permeabledielectric window member coaxially surrounding said inner conductor andprojecting into said field containing region of space, means at theinner end of said window for sealing said window member to said innerconductor, means forming an extension of said outer conductor of saidcoaxial line formed by a portion of one wall of said radial portion ofsaid wave supporting structure disposed overlying an axially coextensiveregion of the inwardly projecting portion of said dielectric windowmember for loading same to maintain the characteristic impedance of theinwardly projecting portion of said coaxial line means, wherebyreflections of wave energy from said window are minimized.

2. The apparatus according to claim 1 wherein said wave supportingregion of space is a portion of a microwave periodic circuit, andincluding means for producing a stream of charged particles adjacentsaid periodiccircuit for cumulative interaction with the fields thereoffor producing an output signal which is coupled from said circuit to aload via said coaxial line.

3. The apparatus according to claim 2 wherein said periodic circuit andsaid particle stream producing means are portions of a voltage tunablemagnetron.

4. The apparatus according to claim 4 wherein said periodic circuit isheavily coupled to the load for loading the periodic circuit andreducing the Q thereof for broadbanding said voltage tunable magnetron.

5. The apparatus according to claim 1 wherein said dielectric windowmember is a ceramic cylinder.

6. The apparatus according to claim 1 wherein said means for sealing theinner end of said window member to said center conductor comprises anannular metallic frame member.

7. The apparatus according to claim 3 wherein said evacuated wavesupporting region of space is contained withinin radial cavity resonatorresonant in the TM mode at the output signal frequency.

8. A composite coaxial line coupling device gas tight wave permeablewindow assembly including, an evacuated wave supporting region of spacedefined between a pair of conductive portions and containing fields tobe coupled to a coaxial line, means forming a coaxial line having aninner and outer conductor for coupling to the fields of said fieldcontaining region of space and having a certain characteristicimpedance, said inner conductor passing through an aperture in one ofsaid field supporting conductor portions and being coupled to the otherconductor portion, means forming a wave permeable dielectric windowmember coaxially surrounding said inner conductor and projecting intosaid field containing region of space, means at the inner end of saidwindow for sealing said window member to said inner conductor, meansforming an extension of said outer conductor of said coaxial linedisposed overlying an axially coextensixe region of the inwardlyprojecting portion of said dielectric window member for loading same tomaintain the characteristic impedance of the inwardly projecting portionof said coaxial line means, and means forming a coupling capacitorstructure disposed at the inner end of said inner conductor of saidcoaxial line means for coupling said inner conductor of said coaxialline means to said other conductor portion which is being coupled tosaid coaxial line, whereby said capacitor structure balances out a smallseries inductive reactance of the inwardly projecting portion of saidcoaxial line to provide a reflectionless window assembly.

References Cited UNITED STATES PATENTS 2,404,086 7/1946 Okress et a1.33333 2,408,271 9/1946 Rigrod et al. 33333 2,557,391 6/1951 Okress etal. 333-33 3,195,010 7/1965 Lock 3l539.53 3,334,266 8/1967 Frutiger3l539.53

HERMAN K. SAALBACH, Primary Examiner.

SAXFIELD CHATMAN, JR., Assistant Examiner.

US. Cl. X.R.

